1. Field of the Invention
The present invention relates generally to systems and methods for measuring time with respect to digital signals, and more particularly to systems and methods for digital time interpolation with respect to digital signals.
2. Related Art
FIG. 1 illustrates a time-varying user signal 104. The frequency of the user signal 104 may be determined by determining the time-difference between consecutive positive zero crossings of the user signal 104 (indicated by A and C). This time-difference is indicated by X.
The time-difference between consecutive zero crossings of the user signal 104 (that is, X) is conventionally determined by using a reference signal 102. Specifically, the time-difference between a positive zero crossing of the user signal 104 (indicated by A) and the immediately following positive zero crossing of the reference signal 102 (indicated by B) is first quantized. Then, the time-difference between the next positive zero crossing of the user signal 104 (indicated by C) and the immediately following positive zero crossing of the reference signal 102 (indicated by D) is quantized. Techniques for using Y and Y' to determine the time-difference between consecutive positive zero crossings of the user signal 104 (that is, X) are well known.
Time-differences between two digitqal signals (such as the reference signal 102 and the user signal 104) are quantized by using digital time interpolation techniques. Many such digital time interpolation techniques currently exist.
For example, vernier devices measure "expanded time" by using a relatively slow time-to-amplitude/amplitude-to-expanded time converter. Alternatively, vernier principal devices measure expanded time by using two clocks of slightly different periods and counting the number of periods until the phases of the two channels are in coincidence. Vernier devices are flawed, however, since they require proportionally longer conversion times for increased time resolution. Consequently, "dead-time" between measurements increases.
Minimizing dead-time is important because dead-time limits the rate at which measurements can be made (instrument dead-time, in some cases, is directly proportional to the time required to interpolate).
Startable ramp interpolators require triggered ramps to start on an asynchronous event and to stop on a synchronized clock edge. Startable ramp interpolators are flawed, however, because they introduce jitter, non-linearities, and reset times. The jitter and non-linearities limit resolution. The reset times contribute to dead-time.
Multiple-phase clock interpolators (such as ring oscillators) require many matched delays (at least one per resolution element). Multiple-phase clock interpolators are flawed since they suffer from square root of N jitter increases (where N is the number of active clock delay elements and jitter is the amount of jitter present in one delay element).